Conventionally known in general as methods of making a ceramic electronic element such as laminated ceramic capacitor are those of sheet process and of printing process. Each of these methods forms a laminate in which a plurality of layers made of a ceramic dielectric powder constituting dielectric layers and a plurality of layers made of a metal paste (electrode paste) constituting inner electrode layers are alternately stacked, fires the laminate, and then providing the laminate with outer electrodes, thereby making a ceramic electronic element.
Employed for forming the dielectric layers is a ceramic article made by mixing the ceramic dielectric powder and an organic binder so as to form a slurry, shaping the slurry into a sheet by a method such as doctor blade method, and drying the sheet as appropriate. The electrode paste used for forming the inner electrodes, on the other hand, is formed by dispersing a metal powder such as nickel powder into an organic binder and an organic solvent so as to yield a paste.
A laminated ceramic capacitor is usually obtained by screen-printing the above-mentioned electrode paste onto a surface of the sheet-like ceramic article, drying the organic solvent contained in the electrode paste, then stacking a plurality of such articles, press forming the articles so as to yield a laminate, forming the laminate into a chip, and firing the chip.
Meanwhile, the electrode paste has a sintering start temperature lower than that of the ceramic article and dielectric paste. Therefore, when the laminate is heated for firing, the electrode paste starts sintering earlier than the ceramic article does. As a result, the electrode paste is fired in excess, and thus aggregates in a state where metal components of the electrode paste are unevenly distributed. Therefore, many discontinuous parts occur in the inner electrode layers in the fired laminate, whereby the continuity of layers may deteriorate remarkably. This may reduce the substantial opposing area between the inner electrode layers, thereby decreasing the electrostatic capacity of the laminated ceramic capacitor produced from the fired laminate.
Therefore, it has generally been practiced that a powder material (common material) of the same species as that of the ceramic powder contained in the dielectric paste is added to the electrode paste, so as to allow the firing start temperature of the electrode paste approach that of the ceramic article, and the shrinkage ratio of the electrode paste at the time of firing to approach that of the ceramic article. Since the common material is an insulating material, however, the function of the inner electrode layer as an electrode deteriorates more as a greater amount of the common material is added to the electrode paste. Therefore, from the viewpoint of electrode characteristics of the inner electrode layer, it is preferred that the amount of addition of the common material be as small as possible.
A technique in which a metal resinate is added to the electrode paste in order to suppress the above-mentioned decrease in electrostatic capacity is disclosed in Japanese Patent Application Laid-Open No. HEI 7-176448, for example. In the technique disclosed in the above-mentioned publication, fine metal particles of the metal resinate are generated between the metal components of the electrode paste at the time of firing the laminate, whereby the electrode function of the inner electrode layer hardly deteriorates even when the metal resinate is added to the electrode paste. Also, adding the metal resinate to the electrode paste can raise the sintering start temperature of the electrode paste. Therefore, the use of the electrode paste to which the metal resinate is added as disclosed in the above-mentioned publication can improve the continuity of the inner electrode layers while restraining the function of the inner electrodes as an electrode from deteriorating.
However, the following problem exists in the above-mentioned electrode paste employed in a ceramic electronic element. Namely, the metal resinate greatly shrinks at the time of firing, since the volume ratio of metal components is low therein. The shrinkage ratio of the metal resinate is much greater than that of the ceramic dielectric powder. Therefore, when only such a metal resinate is added to the electrode paste by a large amount in order for the sintering start temperature of the electrode paste to approach that of the ceramic article, the above-mentioned difference in shrinkage ratio lowers the adhesion between the dielectric layers and inner electrode layers, thereby deteriorating characteristics of the electronic element.
In view of the problem mentioned above, it is an object of the present invention to provide a ceramic electronic element having improved the continuity of inner electrode layers while suppressing the decrease in adhesion between its dielectric layers and inner electrode layers and the deterioration in functions of the inner electrode layers, and a method of making the same.